The present invention relates to an aluminum electrolytic capacitor used in various electronic equipment and particularly relates to an aluminum electrolytic capacitor realizing high ripple current-carrying capacity and high reliability performance and a manufacturing method thereof.
Smaller sizes and enhanced reliability are required of electronic equipment nowadays. Therefore, aluminum electrolytic capacitors must be smaller in size, larger in ripple current-carrying capacity and more reliable in performance. FIG. 6 is a cross-sectional view of a prior art aluminum electrolytic capacitor. As shown in FIG. 6, the anode foil 1 and cathode foil 2, are each connected with the aluminum part 3a of the lead 3, and wound together with the separator 4 sandwiched therebetween, to form a capacitor element 5. Capacitor element 5 is placed with a driving electrolyte (not shown in FIG. 5) in a bottomed tubular metal case 6 and an opening of the metal case 6 is sealed with a sealing material 7, thus completing an aluminum electrolytic capacitor.
With the prior art aluminum electrolytic capacitor as shown in FIG. 6, an oxide film la with a high melting point and high insulating properties is formed on the surface of the anode foil 1, as shown in FIGS. 7a and 7b. Therefore, it has been impossible to use a conventional electrical or thermal process welding method, to weld a lead connecting area 1b of the anode foil 1 with the aluminum part 3a of the lead 3 because of the oxide film 1a. 7(b) is first destroyed and welded using such special connecting methods as an ultrasonic welding method, a swaging joint method and a cold welding method as shown in FIG. 8 to FIG. 10, respectively. (See Japanese Laid Open Patent Application No. H7-235453, for example).
More specifically, the ultrasonic welding method of FIG. 8 is a method, whereby the oxide film la is destroyed by ultrasonic energy and then both the anode foil 1 and lead 3 are joined together. The swaging joint method of FIG. 9 is a method whereby the oxide film 1a is destroyed by a swaging needle and then anode foil 1 and lead 3 are joined together. And the cold welding method of FIG. 10 is a method, whereby the oxide film 1a is destroyed by a pressing pressure received from a press tip. These methods aim to join the lead connecting area 1b of the anode foil 1 with the aluminum part 3a of the lead 3 by bringing both into a direct contact with each other.
With the foregoing prior art electrolytic capacitor, the lead connecting area 1b of an aluminum layer of the anode foil 1 (referred to as aluminum 1b hereafter) and the aluminum part 3a of the lead 3 (referred to as aluminum 3a hereafter) are connected with each other with the oxide film 1a still existing therebetween as shown in FIG. 7(b).
Therefore, no matter how hard an attempt has been made to destroy the oxide film 1a by employing the ultrasonic welding method, the swaging joint method, or the cold welding method, there still remains the highly insulating oxide film 1a between the aluminum 1b and the aluminum 3a, as shown in FIG. 7(b). Accordingly, a perfect connecting condition cannot prevail over the entire contact area between the aluminum 1b and the aluminum 3a, thus creating a resistance therebetween and presenting a problem. The problem of such an imperfect connection as above tends to be multiplied as an aluminum electrolytic capacitor is made smaller in size and larger in capacitance.
More specifically, when an aluminum electrolytic capacitor is made smaller in size, an anode foil of larger capacitance must be used. When an anode foil is made larger in capacitance, the surface roughness of the anode foil generally increases, resulting in a condition where minute recesses and projections are present throughout the surface. When this anode foil of large capacitance is used and the connection between the anode foil 1 and the lead 3 is established by the use of the foregoing ultrasonic welding method, swaging joint method or cold welding method, fine particles of the oxide film 1a, produced by the destruction of the oxide film 1a, remain between the aluminum 1b and the aluminum 3a in a substantial quantity. As a result, only a partial connection is established between aluminum 1a and 3a. Thus, a problem of imperfect connection due to a substantial number of crevices created between the aluminum 1b and the aluminum 3a occurs.
When an aluminum electrolytic capacitor is produced under the condition that the aluminum 1b and the aluminum 3a are connected with each other imperfectly as described above, a resistance at the junction increases to a magnitude that a large ripple current is incapable of flowing through the capacitor. Thereby a restriction to the permissible ripple current, which is a very important electrical property of an aluminum electrolytic capacitor, occurs.
With respect to the reliability of an aluminum electrolytic capacitor using a large capacitance anode foil for miniaturization, existence of a substantial number of crevices between the aluminum 1b and the aluminum 3a together with ripple currents and hot or cold stresses applied to the capacitor for a long period of time, lead to forming of cracks on the surface of the junction. And, therefore, it is anticipated that the dangers of the aforementioned crevices when exposed externally will multiply.
An electrolyte, with which the capacitor element 5 is impregnated, infiltrates into the numerous crevices exposed externally and the infiltrated electrolyte changes the surface of the aluminum inside of the crevices into an electrically insulating oxide aluminum film by an oxidizing action thereof. As a result, the junction between the aluminum 1b and the aluminum 3a experiences an increase in resistance and near the extreme ends the resistance has an appearance of being open-circuited, thereby seriously damaging the reliability of the aluminum electrolytic capacitor.
Additionally, when similar miniaturization of a high voltage aluminum electrolytic capacitor is pursued, the gravity of the problem as described above is multiplied. More specifically, an anode foil of a high withstand voltage is required for the high voltage aluminum electrolytic capacitor. But, due to a substantial thickness of the oxide film on the anode foil, debris of an oxide film 1a created by the destruction of oxide film 1a remains thick between the aluminum 1b and the aluminum 3a when compared with the anode foil 1 for a low operating voltage. This results in an increase of the seriousness of the aforementioned problem.
The present invention deals with the problems involved with prior art aluminum electrolytic capacitors and aims to provide an aluminum electrolytic capacitor realizing a reduction in size, a high ripple-current carrying capacity, and exhibiting low resistance characteristics without a loss in performance, such as high quality and high reliability and provides a manufacturing method thereof.
In order to deal with the foregoing problems, an aluminum electrolytic capacitor of the present invention uses an electrode foil, on part of a surface free of an oxide film and connected with an external lead. And, the present invention proposes a manufacturing method for making these capacitors.
Accordingly, since the lead is connected to an area on the surface of the anode foil where no oxide film exists, no broken pieces of the oxide film produced at the time of connection exist between the foil""s aluminum and the lead""s aluminum. As a result, not only the conventional ultrasonic welding method, sawing joint method and cold welding method but also an electrical and gas welding method such as a resistance welding method, arc welding method and a thermal welding method such as a laser welding method and the like can be used, which before now have been impossible to use. The contact resistance at the lead connecting area is further reduced and a better connection is made, thereby enhancing the quality/reliability of the capacitor and enabling the realization of an aluminum electrolytic capacitor with a smaller size and a higher ripple-current carrying capability with lower resistance.
As described above, the aluminum electrolytic capacitor of the present invention has the lead thereof connected to the surface of the anode foil at a place where no oxide film exists and, therefore, the broken pieces of the oxide film produced at the time of joining and existing between the anode foil""s aluminum and the lead""s aluminum are not present, thereby realizing an almost perfect joint between the anode foil""s aluminum and the lead""s aluminum. As a result, the resistance between the anode foil and the lead is further reduced when using an anode foil of large capacity and a high withstand voltage. And the aluminum electrolytic capacitor for realizing further enhancement of the quality/reliability, a reduction in size, and higher ripple-current carrying capacity and lower dissipation can be obtained.